Deep Subwavelength-Scale Metal–Insulator–Metal Plasmonic Disk Cavities for Refractive Index Sensors

We propose an ultrasmall plasmonic cavity index sensor based on double metal disks, in which a plasmonic mode with a wavelength (λ
₀
) of 1550 nm can be confined. By carefully reducing the radius (R) and gap thickness (t), the cavity can be reduced to a size of R = 744 nm and t = 20 nm, achieving a deep subwavelength-scale sensing volume of 0.0093 λ
₀
³
, while keeping the same wavelength. The figure of merit of the index sensor has a maximum value of 80 with a high sensitivity (1160 nm per refractive index unit) and narrow linewidth (14.8 nm).